Electrorheology is a phenomenon in which the rheology of a fluid is modified by the imposition of an electric field. Fluids which exhibit significant changes in their properties of flow in the presence of an electric field have been known for several decades. The phenomenon of electrorheology was reported by W. M. Winslow, U.S. Pat. No. 2,417,850, in 1947. Winslow demonstrated that certain suspensions of solids in liquids show large, reversible electrorheological effects. In the absence of an electric field, electrorheological fluids generally exhibit Newtonian behavior. That is, the applied force per unit area, known as shear stress, is directly proportional to the shear rate, i.e., change in velocity per unit thickness. When an electric field is applied, a yield stress appears and no shearing takes place until the shear stress exceeds a yield value which generally rises with increasing electric field strength. This phenomenon can appear as an increase in viscosity of up to several orders of magnitude. The response time to electric fields is on the order of milliseconds. This rapid response, characteristic of electrorheological fluids, makes them attractive to use as elements in mechanical devices.
A complete understanding of the mechanisms through which electrorheological fluids exhibit their particular behavior has eluded workers in the art. Many have speculated on the mechanisms giving rise to the behavior characteristics of electrorheological fluids.
A first theory is that the applied electric field restricts the freedom of particles to rotate, thus changing their bulk behavior.
A second theory ascribes the change in properties to the filament-like aggregates which form along the lines of the applied electric field. The theory proposes that this "induced fibrillation" results from small, lateral migrations of particles to regions of high field intensity between gaps of incomplete chains of particles, followed by mutual attraction of these particles. Criticism of a simple fibrillation theory has been made on the grounds that the electrorheological effect is much too rapid for such extensive structure formation to occur; workers in the art have observed a time scale for fibrillation of approximately 20 seconds, which is vastly in excess of the time scale for rheological response of electrorheological fluids. On the other hand, response times for fibrillation on the order of milliseconds have been observed.
A third theory refers to an "electric double layer" in which the effect is explained by hypothesizing that the application of an electric field causes ionic species adsorbed upon the discrete phase particles to move, relative to the particles, in the direction along the field toward the electrode having a charge opposite that of the mobile ions in the adsorbed layer. The resulting charge separation and polarization could lead to "dipole" interactions and fibrillation.
Yet another theory proposes that the electric field drives water to the surface of discrete phase particles through a process of electro-osmosis. The resulting water film on the particles then acts as a glue which holds particles together. If correct, then a possible sequence of events in fibrillation would be: ionic migration, subsequent electro-osmosis of moisture to one pole of the particle (presumably the cationic region) and bridging via this surface supply of water. However, the advent of anhydrous electrorheological fluids means that water-bridging is not an essential mechanism and may indeed not be operative at all.
Despite the numerous theories and speculations, it is generally agreed that the initial step in development of electrorheological behavior involves polarization under the influence of an electric field. This then induces some form of interaction between particles or between particles and the impressed electric or shear fields which results in the rheological manifestations of the effect. See Carlson, U.S. Pat. No. 4,772,407; and Block et al "Electro-Rheology", IEEE Symposium, London, 1985. Despite this one generally accepted mechanism, the development of suitable electrorheological fluids and methods of improving the same remains largely unpredictable.
The potential usefulness of electrorheological fluids in automotive applications, such as vibration damping, shock absorbers, or torque transfer, stems from their ability to increase, by orders of magnitude, their viscosity upon application of an electric field. This increase can be achieved with very fast (on the order of milliseconds) response times and with minimal power requirements.
Although ER-fluids have been formulated and investigated since the early 1940's, basic limitations have prevented their utilization in practical devices. The most restrictive requirements are (1) that the suspensions be stable over time; i.e., that the solid particles either remain suspended in the liquid or be readily redispersed if sedimentation occurs and (2) service and durability of the suspensions can be achieved outside the temperature range of 0.degree.-100.degree. C. This latter requirement is particularly restrictive in that most fluid compositions require water as an ER "activator" so that in completely nonaqueous systems the ER-effect is entirely absent or so small that it is not effectively useful.
An object of this invention is to formulate a stable, substantially water-free, or nonaqueous ER-fluid with improved properties.